I just built a website, http://PCBShopper.com, that lets you enter the specs for your PCB (dimensions, layers, preferred solder mask color, and quantity) and shows you a list of manufacturers and their prices (in US dollars, including shipping to the US). Currently, it includes 9 different manufacturers, and I hope to add more as time goes on.

PCBShopper also has a page comparing free PCB CAD programs. Since several of them are crippled versions of commercial programs, the comparison includes the limitations. For both manufacturers and CAD programs, users can write ratings and reviews, Amazon-style.

I hope PCBShopper will be useful for my fellow hobbyists. I am not affiliated with any manufacturers or CAD software developers.